java.lang.NullPointerException com.lightning.service.impl.CategoryServiceImpl.listCategories(CategoryServiceImpl.java:18) com.lightning.controller.CategoryController.listCategories(CategoryController.java:20) sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) java.lang.reflect.Method.invoke(Method.java:498) org.springframework.web.method.support.InvocableHandlerMethod.invoke(InvocableHandlerMethod.java:215) org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:132) org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:104) org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:743) org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:672) org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:82) org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:933) org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:867) org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:951) org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:842) javax.servlet.http.HttpServlet.service(HttpServlet.java:635) org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:827) javax.servlet.http.HttpServlet.service(HttpServlet.java:742) org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
百度了一下，發現很多人都犯過這個錯誤。 原因是service.impl.XXServiceImpl下面，忘記給mapper/dao 寫@Autowired了。
自己寫代碼總是能踩到很多坑。所以光看不行，還要多寫。
